The mother of Mikhail Khodorkovsky – the billionaire oligarch locked up for defying Vladimir Putin – has appealed to the foreign secretary, David Miliband, to raise her son's case during his high-profile visit to Russia tomorrow Monday.

In an interview with the Observer, Khodorkovsky's mother, Marina Filipovna, 74, said she was "deeply concerned" by the US administration's new hands-off approach to Russia – and said it was essential that Britain kept human rights at the forefront of its dialogue with the Kremlin.

"It's very important that western leaders keep raising my son's case," she said. Asked whether the Russian government was likely to heed criticism from Miliband, given London and Moscow's recent fraught relations, she replied: "Foreign governments have enough levers that they can use."

Miliband is arriving in Moscow tomorrow on his first visit to Russia as foreign secretary. The date coincides with the third anniversary of the poisoning in London of former Russian agent Alexander Litvinenko, which thrust British-Russian relations into a period of turmoil from which they have not yet fully recovered.

Speaking from a boarding school for orphans funded by Khodorkovsky outside Moscow, Filipovna said she was "alarmed" by US secretary of state Hillary Clinton's recent visit to Russia and the apparent downgrading of human rights issues by President Barack Obama. "During a radio interview Clinton was twice asked about my son's case. She refused to comment. This is very worrying," she said. Once Russia's richest man, and chief executive of the country's biggest oil company Yukos, Khodorkovsky was dramatically arrested in October 2003 on the runway of a Siberian airport.

The criminal case against him was widely seen as punishment for his decision to fund opposition parties and to challenge the might of then-president Putin. Khodorkovsky was later jailed for nine years, reduced to eight, for fraud and other offences.

In 2007, prosecutors filed fresh charges against Khodorkovsky and his business partner Platon Lebedev. For the past seven months both defendants have appeared in court in Moscow, sitting in a heavily-guarded bulletproof glass cage dubbed the "aquarium".

Supporters say this second trial is groundless, contradictory and increasingly surreal. This second case – on charges suspiciously similar to the original ones – has caused consternation in the European Union, with the Council of Europe's rapporteur, Sabine Leutheusser-Schnarrenberger, recently declaring she was "perplexed" by the Russian government's decision to launch fresh proceedings at the moment when Khodorkovsky was coming close to parole. The European Court of Human Rights will soon rule on the question of whether the Russian state was entitled to grab Yukos's assets.

Asked why the Kremlin had singled out her son for punishment, Filipovna said: "Obviously there was some sort of political motivation behind it, even though my son never harboured any political ambitions. But more importantly, people were trying to get hold of his property and steal his money."

Khodorkovsky was surviving his ordeal surprisingly well, she said. "His spirits are always good. He's never complained to me." She conceded he looked "extremely pale", but added: "This isn't really surprising, given the fact he's spent the last six years in jail and never gets any fresh air."

Khodorkovsky recently described how he spends four hours every day travelling to court by truck through Moscow's notorious traffic, sitting in a cramped "metal booth". Once the world's 16th richest man, the tycoon has porridge and coffee for breakfast. He gets a shower once a week. When not attending court he spends 23 hours a day locked up in his cell. He is allowed just one hour's recreation in an "outdoor" cell covered with metal netting.

Russia's president Dmitry Medvedev has repeatedly called for an end to his country's "legal nihilism". Some optimists believe he is sincere in his stated desire to reform the country. Khodorkovsky's second trial is widely seen as emblematic: observers ask if Russia wants to re-engage with other countries or continue its Putin-era slide towards autocracy and contempt for the rule of law. Last week Khodorkovsky responded to a much-discussed blog by Medvedev, entitled "Go Russia!", which called for the country's modernisation. Writing in the Russian daily Vedomosti, Khodorkovsky dismissed Medvedev's article as "dispiriting" and said it was impossible to modernise Russia without first dismantling its authoritarian system.

"The issue here isn't just that Russia doesn't meet many of the customary humanitarian requirements for any country that wishes to consider itself modern and European… but rather that the infamous 'vertical of power' is spectacularly inefficient," he wrote. Robert Amsterdam, Khodorkovsky's international lawyer, said: "For Russia this second trial is a grotesque embarrassment." Asked if – as seems likely – his client would be jailed again at the end of the trial, he said: "I see beyond the verdict. The trial isn't real any more. It's a political case. Ultimately there will be a political solution."

Striking a positive note, Amsterdam said: "Medvedev is reading what Khodorkovsky is writing. This is a sea change. Rather than receding into the fog of history, Khodorkovsky is continuing to be part of the daily fabric of Moscow."

If convicted on the latest charges of fraud and money laundering, Khodorkovsky faces up to 22 years in jail. Marina, who is 74, said the family was allowed only two visits to see him a month.

"The judge is not stupid. He understands what's going on," Marina said. 'The big question is whether he's going to be able to decide things for himself.'